L0301P86 - Interactions Between Bacteria and Human Host
Interactions: Humans and Microorganisms *Can give rise to states of: **health **disease ***any harmful alteration to physiological/metabolic state of the host **infectious disease ***alterations caused by microorganism or its products (toxins) **epidemics ***rapid rise of disease incidence in a locality **death *balance between pathogen attack and host defence is required to keep the host alive *normal flora can become pathogenic - opportunistic *microorganisms in the environment can also cause infections Normal Flora *acquired rapidly **during and shortly after birth *changes continuously throughout life *1014 bacteria associated with areas exposed to external environment Human Skin *has various microenvironments **is relatively dry, slightly acidic **primary source of nutrition is dead cells **moist areas ***axillae, perineum, between toes, scalp *composition varies from site to site according to the character of the microenvironment *normal flora of the skin consists mostly of large populations of Gram-positive bacteria **e.g.: Corynebacteria, Staphylococci *can also be found below skin surface in hair follicles, sweat glands, sebaceous glands **e.g.: Propionibacterium acnes Nose and Oral Cavity *abundant and diverse *must be able to resist mechanical removal *oral flora is involved in dental caries and periodontal disease *bacterial flora that reside include both Gram positive and negative bacteria **e.g.: Staph. aureus, Strep. pyogenes, Neisseria meningitidis Mouth *complex bacterial habitat *potential microbial nutrients: saliva and food *saliva is not a good culture medium **contains antibacterial substances ***e.g. lysozymes *organisms must therefore attach to surfaces to resist the flushing action of the oral cavity **e.g.: gums and teeth Dental Plaque *found on teeth and gingival (gum) crevices *build up of bacteria and material on teeth anchored in a polysaccharide matrix **known as a biofilm *main species present: **Strep. mutans and Strep. sanguis *depleted of oxygen - an anaerobic microenvironment Dental Caries *involves the destruction of the enamel due to bacterial activity *bacteria in dental plaque convert sucrose (CHO) to lactic acid on the tooth surface where it can attack/break down the tooth *sucrose-heavy diet predisposes host to dental caries *low sucrose diet reduces build up of plaque *e.g.: Streptococcus mutans **produces levan (polymeric fructose) Respiratory Tract *nasal mucus contains lysozymes Upper Respiratory Tract *many species as normal flora *similar to the normal oral cavity *both Gram positive and negative bacteria *some may be opportunistic pathogens **e.g.: Streptococcus pyogenes, Haemophilus influenzae Lower Respiratory Tract *trachea, bronchi, bronchioles and alveoli do not normally have microbiota *microorganisms removed by: **mucus ***continuous stream generated by ciliated epithelial cells **alveolar macrophages ***phagocytic action Gastrointestinal Tract *varying pH and nutritional sources *stomach acidity, bile and pancreatic secretions inhibit growth of microbiota Stomach *hostile environment - acidic pH (2) *“barrier” to entry to intestinal tract *gastric mucosa **acid-tolerant Lactobacilli and Streptococci Stomach Ulcers *originally thought to be a “stress” disease *actually infectious - Helicobacter pylori **flagellated rod bacteria **found in half the human population **many infected show no symptoms **other factors are involved that drive the disease process Intestines *distance from stomach = pH increase (4-5) = bacterial numbers increase Small Intestine *duodenum lightly colonised **104 organisms/gram *jejunum low levels **106 organisms/gram *ileum moderate levels **109 organisms/gram) **similar to colon as pH (7) more alkaline **i.e. Gram positive and negative bacteria Large Intestine *enormous numbers of bacteria **1011-1012/gram of intestinal contents ***~30% of faecal volume, >400 species ***e.g. Candida albicans, protozoa reside as harmless commensals *oxygen depleted **strictly anaerobes predominate (95-99%) **e.g. bacteroides (strict) E. coli (facultative) Genitourinary Tract *upper tract (kidneys, bladder) and upper urethra usually sterile *lower urethra **does have some bacteria from skin, colon, vagina (in women) **frequently washed out by urinary flow **in women, vagina complex microbiota ***most abundant is lactobacilli The Disease Process Disease *when the body no longer functions properly *various reasons why: **genetic, metabolic, deficiency, infection Infection *bacteria or other microorganisms penetrate the host defences *invade tissues, multiply and cause damage Definitions Infectious Disease Process *existence **of the specific pathogenic microorganisms *reservoir **where microorganism exists and persists *mode of transmission *means of entry *host susceptibility Pathogens vs. Normal Flora *pathogens can cause disease as they express virulence determinants whereas normal flora do not *virulence factors mediate: **adhesion (ligands for cells) **anti-phagocytic (capsule) **damage (toxin) *to cause disease most pathogens must: **enter the body **colonise the host **evade host defences **multiply and disseminate **cause damage to host Microorganism Pathway Entry and Exit Sources of Infection *source: where infectious agent is acquired *inanimate **food, soil and water **inanimate objects (fomites) *animate **animals (zoonosis) **insects **human Transmission Between Humans Barriers to Entry *physical barriers **skin (high salt, fatty acids) **stomach (acid) **lower respiratory tract (mucous, cilia) **gastrointestinal tract (peristalsis) **soluble mediators eg. Lysozyme *innate immune responses *adaptive immune responses   Initial Barrier - Epithelial Layer *all pathogens must develop a means of getting past the epithelium *route used is typically characteristic of a particular pathogen i.e. they can enter via: **skin **gastrointestinal tract **respiratory tract **genitourinary tract *all pathogens also must develop a means to exit and transmit to the next host Routes of Infection Skin *dead skin, shed constantly **resident or transient bacteria *hair follicles, sebaceous glands *cracks, splits due to swelling or drying *crusts (scabs), exudates, wounds, abrasions, burns Common Bacterial Diseases (of/via skin) *skin infections **acne, impetigo (school sores) **folliculitis, boils (furuncles), carbuncles **Staphlycoccal scaled skin syndrome **bairnsdale ulcer **cellulitis, erysipelas *bacteria involved **Staphylococcus aureus **Streptococcus pyogenes **Mycobacterium ulcerans *wounds **skin flora - S. pyogenes, S. aureus **soil-contaminated ***Clostridium perfringens - gas gangrene *surgical **S. aureus, S. epidermidis, group A Streptococci (GAS) **necrotising fasciitis: “flesh eating bacteria” *animal and human bites **pasteurella multocida, S. aureus, oral streptococci, Clostridium tetani *burns **P. aeruginosa, S. aureus Respiratory *respiratory or saliva spread **inhalation ***aerosols ***dust particles ***spores *rapid spread *increase in secretions, sneezing, coughing, expectoration (spit form throat or lungs) *fomites: handkerchiefs, hands, other objects Common Bacterial Diseases (via RT) *Whooping cough - Bordetella pertussis *Tuberculosis - Mycobacterium tuberculosis *Strep throat - Streptococcus pyogenes *Pneumonia - Streptococcus pneumoniae Faecal-Oral *ingestion *survival *organisms have special attachment mechanisms *GI infections are prevalent where **poor hygiene and public health *inadequate sewerage disposal *e.g. typhoid, cholera Common Bacterial Diseases (via GIT) *Cholera - Vibrio cholerae *Diarrhoea - multiple causes **E. coli **Salmonella Enteritidis **Campylobacter jejuni *Shigellosis - Shigella sonnei *Dysentery - Shigella dysenteriae Genitourinary Tract *lower urethra **contaminated with normal flora *females **short urethra **ascending infection commonly infects the bladder *catheter **hospital acquired ***normal flora, antibiotic resistant strains Common Bacterial Diseases (via GUT) *cystitis - urinary tract infections (UTI) **E. coli **Staphylococcus saprophytic *in hospitals, some UTIs also due to: **Pseudomonas **Enterococcus **Candida albicans (a yeast) *pyelonephritis - kidney infection **E. coli causes about 90% Genital *intimate contact - intercourse *childbirth **genital tract infections acquired by the mother can be acquired by the baby at birth via birth canal *congenital **infections acquired by the foetus in utero ***i.e. infection transverse the placenta Common Sexually Transmitted Diseases *Gonorrhoea - Neisseria gonorrhoeae **rarely any signs and symptoms in women so spread is very easy **nowadays often antibiotic resistant *Chlamydia - Chlamydia trachomatis Other Bacterial Diseases *perinatal infections - vaginal delivery **conjunctivitis ***C. trachomatis, N. gonorrhoeae **pneumonia ***Group B streptococci, C. trachomatis **meningitis ***Streptococcus agalactiae *congenital **Listeriosis - Listeria monocytogenes